Monitoring sequestration related mineralization processes in basalts using marine controlled source electromagnetics

Carbon storage in basaltic formations is regarded as one of the safest and most stable long-term sequestration strategies, because mineralization immobilizes CO₂ in solid form. The global abundance of basalts also implies substantial storage potential at very large scales, especially in marine settings where potential usage conflicts would be avoided. However, reliable monitoring is required to confirm storage performance and integrity. In the marine environment, controlled-source electromagnetics (CSEM) is sensitive to subsurface resistivity variations and, therefore, offers a potential means to monitor resistivity changes associated with CO₂ injection and subsequent mineralization processes.

In the PERBAS project we investigate the basalt formations of the Vøring Plateau, offshore Norway. There, borehole data show that several hundred-meter-thick target basalt units consist of interchanging layers of basalt and sedimentary layers with individual thicknesses of several meters. While resolving these layers individually is beyond the resolution of CSEM experiments, the strong layering of resistors (basalts) and conductors (sediments) should result in a significant anisotropic effect on CSEM data.

For a modeling study, we consider three different scenarios: (1) the pre-injection stage represented by the background resistivity distribution, (2) the injection stage, where the CO2 is mainly propagating in the sedimentary layers and (3) the post-injection stage, where the CO2 might have migrated into the basalt layers to form stable minerals. Based on resistivity-logs from boreholes, we constructed a simple 1D anisotropic pre-injection model (stage 1). For the injection scenario before mineralization (stage 2), Archie’s law was used to estimate resistivity increases in sediment layers. For the mineralization scenario (stage 3), resistivity changes in the basalt were estimated based on the current literature and in-situ information from drill holes in the working area. We then calculated time-domain CSEM electric field responses for both, inline and broadside source–receiver configurations, analyzed electric fields changes between the different scenarios and configurations, and evaluated how the electrical anisotropy is affected and best resolved in terms of CSEM measurements and measurements configurations.

The modelling indicates that both inline and broadside configurations are sensitive to CO₂ mineralization (stage 3) and yield clear difference between pre- and post-injection. However, only the broadside configuration can clearly distinguish pre- and post-injection states where CO₂ stays unmineralized in the sediments (stage 2). In summary, our results suggest that measuring both inline and broadside components in CSEM measurements can yield insights into monitoring mineralization processes in anisotropic basalt–sediment systems.